1. Field of the Invention
This invention relates generally to a cascaded difference frequency generator using a resonant structure, and more specifically, to a resonant cascaded difference frequency generator capable of improving the conversion efficiency of second-order nonlinear optical phenomena by resonating the second harmonic wave of a pump light.
2. Discussion of Related Art
Since an optical phenomenon related to second-order nonlinearity was first discovered, one of the major concerns in the long history of research on the optical phenomenon has been to improve the conversion efficiency in the phenomenological process. A lot of attempts to obtain the higher conversion efficiency include searching materials with higher nonlinear structures, composing new materials, finding new methods of phase-matching in various manners, and fabricating a nonlinear material into an optical waveguide form to increase the interaction of three light waves to be mixed, etc.
The difference frequency generation is to obtain a newly converted idler light (Wi) corresponding to a difference frequency between a signal light (Ws) and a pump light (Wp) by mixing the two lights in a second-order nonlinear medium. In this case, the newly converted light is obtained through the interaction of three wave mixing in virtue of the second-order nonlinearity (see J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, “Interactions between light in a nonlinear dielectric,” Phys. Rev., vol. 127, pp. 1918–1939, 1962). Since the second-order nonlinear process of generating difference frequency, as described above, is an all-optical interaction excluding electrical interactions, it can be useful for wavelength conversion in ultra-high speed optical communication. In addition, since the converted light generated by the process has a conjugated phase due to the second-order nonlinear interaction, it can be useful for dispersion compensation in the ultra-high speed optical communication as well.
Precisely speaking, the direct difference frequency generation is to obtain a converted light of a difference frequency (Wi=Wp−Ws) between the signal light (Ws) and the pump light (Wp) through directly putting the two lights into a second-order nonlinear medium. On the other hand, the cascaded difference frequency generation is to obtain a converted light (Wi) through putting the signal light (Ws) and the pump light (Wp) into a second-order nonlinear medium, generating the second harmonic wave (Wp+Wp=2Wp) of a doubled frequency, and then generating the cascaded difference frequency in virtue of a simultaneous interaction between the second harmonic wave the pump light and the incident signal light (Wi=2Wp−Ws) (see, B. Zhou, C. Q. Xu, and B. Chen, “Comparison of difference frequency generation and cascaded based wavelength conversion in LiNbO3 quasi-phase-matched waveguides,” J. Opt. Soc. Am. B. 20, pp. 846–852, 2003). In a wavelength band adjacent to a communication wavelength range of optical communication, if a wavelength of the converted light (Wi) is not significantly different from that of the incident signal light (Ws), i.e. Wi≈Ws, the direct difference frequency generation essentially needs a completely new light-source equivalent to the frequency Wp of the pump light. In the meanwhile, since the cascaded difference frequency generation can be performed even though the frequency of the pump light is not significantly different from that of the signal light, i.e. Wp≈Wi≈Ws, it does not necessitate another light-source corresponding to the frequency of 2Wp. Thus, it has an advantage that the same kind of a light-source within the communication wavelength band can be used as a source of the pump light.
However, in the case of the cascaded difference frequency generation, if the incident pump and signal lights pass through the nonlinear medium, the pump light first generates the second harmonic wave by the nonlinear process, and then this second harmonic wave of the pump light is coupled to the signal light by the nonlinear process to generate the converted light. In other words, the cascaded difference frequency generator generates the converted light by the (X(2):X(2)) processes in virtue of cascaded second-order nonlinear interactions. Therefore, it has a problem that its conversion efficiency is degraded, and therefore a large amount of the signal light, the pump light, and the second harmonic wave of the pump light do not participate in the nonlinear interactions, but are transmitted outside together with the converted light.